A variety of gas mixtures originating from refineries and natural gas contain hydrogen sulphide (H.sub.2 S) which must be eliminated as completely as possible by recovering sulphur so as to be able to discharge into the atmosphere a gas which satisfies regulations fixed by various industrialised countries. While these regulations were relatively easy to satisfy in the very recent past since they were relatively lenient, this is not the current case particularly in countries such as the United States of America or some states in that country and a gas mixture containing more than 500 parts per million (ppm) of sulphur dioxide and hydrogen sulphide can no longer be discharged into the atmosphere.
As long ago as 1970, the Institut Francais du Petrole developed a process for eliminating hydrogen sulphide as elemental sulphur using the Claus reaction under conditions which were particularly favourable as regards ease of operation and the relatively low cost of the process. The process, which is known under the trade name Claupol 500, employs a homogeneous catalyst in a solvent to transform hydrogen sulphide to elemental sulphur at a temperature of less than 160.degree. C. by reaction with sulphur dioxide (SO.sub.2). After treating the acidic gas containing hydrogen sulphide in a conventional Claus unit, it produces an overall sulphur yield of more than 98%. This process is used industrially in more than 30 plants in various countries of the world, among them the United States.
The initial process developed in the 70s has recently been improved, at the beginning of the 90s, and can now produce a sulphur yield which is of the order of 99.5%. The initial process is described, for example, in the article by Y. Barthel et al., published in 1974 in n.degree.136 of the review "Informations Chimie", pages 235 to 238, and in "Petrole et Technique", n.degree.136, October 1993, pages 24 to 39. The improved process normally known as the CLAUSPOL 300 process has been described in the "Revue de l'Institut Francais du Petrole", vol. 49, n.degree.5, September-October 1994, pages 491-493.
Those two processes use a liquid phase essentially constituted by glycol which is circulated as a counter-current to the gas mixture to be purified containing hydrogen sulphide and sulphur dioxide in a ratio of close to 2. Such processes use a catalyst dissolved in glycol, which is selected from alkali or alkaline-earth metal carboxylates.
The prior art is also illustrated in the following patents: French patents FR-A-2 216 223, FR-A-2 532 190, FR-A-2 338 738 and Japanese patent JP-A-54 072 762 (WPI Database, Section Ch, Week 7929, Derwent Publications Ltd., London, GB; AN 79-53653 B).